Hello Peter Denton,
welcome back to the Polycom Community.
Only a unit with a multipoint license can start these. You cannot dial into a Unit with a Multipoint license to host a call on it without the Site (in this example A) actually hosting the participants.
You would need a bride for this instead.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Best Regards
Steffen Baier
Polycom Global Services